With chronological age and chronic exposure to adverse environmental factors, the visual appearance, physical properties, and physiological functions of skin change in ways that are considered cosmetically undesirable. The most notable and obvious changes include the development of fine lines and wrinkles, loss of elasticity, increased sagging, loss of firmness, loss of color evenness (tone), coarse surface texture, and mottled pigmentation. Less obvious, but measurable changes which occur as skin ages or endures chronic environmental insult include a general reduction in cellular and tissue vitality, reduction in cell replication rates, reduced cutaneous blood flow, reduced moisture content, accumulated errors in structure and function, and a reduction in the skin's ability to remodel and repair itself. Many of the above alterations in appearance and function are caused by changes in the outer epidermal layer of the skin, while others are caused by changes in the lower dermis. Regardless of the stimulus for skin damage, when damage occurs, numerous natural and complex biochemical mechanisms are set into motion in attempts to repair the damage.
When the epidermis is injured, the epidermal basal cells respond to the injury by dividing at a more frequent rate. This increase in replication rate results in a more rapid replacement of the damaged epidermis with a new epidermis and stratum corneum, a process referred to as “epidermal cell renewal.” Common examples of injuries which can increase epidermal cell renewal rates include abrasion, chemical damage, pH extremes, excessive sun exposure, or allergic or non-allergic contact irritation. If the injury is too severe, the increased replication will result in a “hyperplastic” epidermis and a thickened, poorly-functioning stratum corneum which is manifested as dry, rough scales. Other common stimuli which induce epidermal cell renewal include physical removal of the stratum corneum (i.e., an example of which is tape stripping, a process where tape is applied to the skin and pulled off, removing the top layer of the stratum corneum with it) and friction (i.e., on the soles and heels of the feet), all processes which result in epidermal hyperplasia. Hydroxy acids and retinoids also induce epidermal hyperplasia at appropriate concentrations, although the mechanisms appear to be different. It is believed by many that hydroxy acids exert their effects by inducing physical exfoliation of the corneum, whereas the retinoids more likely work by interacting with cytoplasmic and nuclear binding receptors to alter gene expression. Schlitz, J. et al. “Retinoic acid induces cyclic changes in epidermal thickness and dermal collagen and glycosaminoglycan biosynthesis rates,” J. Invest. Dermatol. 87:663-667 (1986), describe various effects of retinoic acid on epidermal and dermal biology including cyclical epidermal thickening and hyperplasia, and dermal changes in the rates of biosynthesis of collagen and glycosaminoglycans.
With chronological age and chronic environmental exposure (notably UVA, UVB, and IR radiation), the dermis undergoes changes in structure and function which result in many of the characteristics of aged skin, including loss of elasticity, formation of wrinkles, loss of water-holding capacity, sagging, and poor microcirculation. At the molecular level, these changes have been correlated with biochemical changes in the content and structure of the extracellular matrix to which the major cells of the dermis (i.e., the fibroblasts) reside. Collagen becomes highly cross-linked and inelastic, elastin is reduced in amounts and is incorrectly distributed, and the glycosaminoglycans become reduced in amounts, which results in reduced intercellular water.
As a result of this changed architecture, the normal amounts and distribution of trace metal ions, growth factors, hormones, and cytokines becomes altered which causes the fibroblasts to become metabolically less active or quiescent. Although these cells have natural mechanisms to repair themselves and the matrix in which they reside, with age and too much damage, they are less able to repair the damage, and the condition continues to deteriorate. If the quiescent fibroblasts can be metabolically activated and stimulated to divide, they will synthesize new extracellular matrix and the old, damaged matrix will be enzymatically degraded and replaced. This process of balanced synthesis and degradation is referred to as “dermal remodeling.” The activation process can be accomplished in many different ways, including chemical stimulation by selected hormones, growth factors, cytokines, vitamins, botanical extracts and retinoids, or by increasing the nutrient supply (i.e., blood flow) to the tissue.
Although the mechanisms are not completely understood, it appears that physical or chemical changes to the intact stratum corneum of the skin will result in epidermal basal cell replication and subsequent increases in epidermal cell renewal. If the injury stimulus is too great, the skin will be unable to correct the damage or will “over-respond” in such a way as to cause extensive epidermal hyperplasia and dry, flaky, poorly-differentiated stratum corneum. If the damage stimulus is less and is well controlled, the process of epidermal replacement should result in a healthier, better-functioning epidermis and in a stratum corneum which looks and feels better, has greater capacity to hold moisture, and has fewer surface fine lines.
It is known that damage to the stratum corneum not only sets into motion natural biochemical mechanisms to repair and replace the epidermis, but disturbances in the corneum also stimulate repair and remodeling of the dermis. U.S. Pat. No. 5,720,963 to Smith (“the '963 patent”) teaches that chronic and significant disruption of the skin's water barrier using a combination of cerebrosides, hydroxy acids, and retinoids causes chronic injury to the corneum and results in epidermal and dermal repair of the structurally-deteriorated skin if the disruption is maintained for a sufficient period of time. The mechanisms by which the combination of materials used in the '963 patent cause increased epidermal cell renewal and chronic skin repair are entirely different from those which are involved in the current invention. Although the end benefits to the skin are similar, the '963 patent teaches that water barrier disruption agents such as cerebrosides or organic solvents or detergents, in combination with retinoids or hydroxy acids will disrupt the corneum water barrier and stimulate basal cell replication rates. The Applicant has discovered that chronic stimulation of the normal process of desquamation by activation of at least one endogenous corneum protease will result in increased epidermal turnover and epidermal and dermal remodeling. This is not a chronic injury; instead, it is an increase in the rates of corneum shedding by one of the natural mechanisms; the increased desquamation stimulates the natural mechanisms involved in replacing the lost corneum, a process which results in beneficial skin remodeling.
In a series of peer-reviewed scientific publications, Lundstrom, A. and Egelrud, T. “Cell Shedding from Human Plantar Skin In Vitro: Evidence of its Dependence on Endogenous Proteolysis,” J. Invest. Dermatol. 91:340-343 (1988); Egelrud, T. and Lundstrom, A., “The Dependence of Detergent-Induced Cell Dissociation in Non-Palmo-Plantar Stratum Corneum on Endogenous Proteolysis,” J. Invest. Dermatol. 95: 456-459 (1990); Lundstrom, A. and Egelrud, T. “Stratum Corneum Chymotryptic Enzyme: A Proteinase which may be Generally Present in the Stratum Corneum and with a Possible Involvement in Desquamation,” Acta Dermato-Venereol 71:471-474 (1991); investigators from the Department of Dermatology, University Hospital, Umea, Sweden, demonstrated that human stratum corneum possesses endogenous chymotryptic proteinase enzyme(s) which can be activated in vitro by a combination of the cationic surfactant N,N-dimethyldodecyl amine oxide (DMDAO), the anionic surfactant sodium dodecyl sulfate (SDS), and the chelating agent ethylene diamine tetraacetate (EDTA). Activation of these enzymes resulted in the separation of individual corneum cells (i.e. corneocytes), a process which Egelrud's group has proposed mimics the natural desquamation process. The Egelrud group have described the role of the endogenous chymotryptic proteinase enzyme(s) to be important in natural desquamation of the skin, but their work did not predict that stratum corneum protease activation would enhance the rates of epidermal cell renewal and set up natural epidermal and dermal biochemical mechanisms which could provide benefit to aged or environmentally-damaged skin.
Prior art physiological, chemical, or mechanical methods of increasing stratum corneum renewal rates to achieve benefit such as hydroxy acids, retinoids, barrier disrupters, tape stripping, solvent extraction, etc. all have various drawbacks, such as significant irritation to the skin, skin toxicity, the requirement of high concentrations of expensive ingredients, or of low pH. In addition, all these methods involve the invocation of damage to the skin, which sets up repair mechanisms. For most of these treatments, there will be a period of time, up to several weeks or months, during which the skin becomes irritated and after which tolerance sets in and the symptoms of irritation may decrease and/or cease. When using the method described in the present invention, these drawbacks are not encountered.
Applicant has surprisingly discovered a method of treatment and composition for aged and environmentally damaged skin that enhances the stratum corneum turnover rate by activation of at least one endogenous proteinase. The treatment, which results in skin with improved visual appearance, function, and clinical/biophysical properties, is not known in the prior art. Moreover, the novel method of treatment and compositions of the present invention accomplish this at low concentrations, at a neutral pH, in all the vehicles in which the actives have been tested, and without causing clinical irritation or chronic damage to the skin.